Bull. Geol. Soc. Finland 40, 107—111 (1968)
CHEMICAL COMPOSITION OF BIOTITES AS A GUIDE TO ASCERTAIN
THE ORIGIN OF GRANITES
N. W.
GOKHALE
Karnatak University, Dharwar 3, India
ABSTRACT
Biotites occurring in igneous and metamorphic-metasomatic rocks have been critically studied. The study reveals
a marked difference between the chemical composition of biotites from igneous rocks on one hand and those from
metamorphic-metasomatic ones on the other. It is concluded that the chemical composition of a biotite reflects the
environment of its formation as well as that of the rock containing it. The most frequently occurring ferromagnesian
mineral in granites is biotite. As such it is thought that by processing the chemical data as suggested in this paper
it should be possible to distinguish between granites formed by more than one mode of origin.
Introduction
It has been agreed upon by penologists that
the crystalline rocks have originated under two
environments namely, magmatic and metamorphic-metasomatic. This is especially true of granitic rocks (Gorai 1951, Tertsch 1941,1942, Turner 1951). Various criteria have been proposed
to indicate the magmatic and metamorphicmetasomatic origin of crystalline rocks. One
such approach is to study the minerals of these
rocks. In this paper emphasis is laid on granites,
since the origin of these rocks is a matter of
great dispute. Besides other minerals, biotite is
one which is more often studied, since it is the
most frequently occurring ferromagnesian mineral in granites. Heinrich, Nockolds, Hall, Peikert and others have studied the chemical compositions and/or the R.I. of the biotites occurring
in various rocks and they have drawn certain
conclusions. Heinrich has interconnected the
chemical composition of the biotite and its geological occurrence. Based on his studies, he has
recognised eight groups of rocks namely —
1.
2.
3.
4.
5.
6.
granite pegmatites.
granites, quartz-monzonites, granodiorites.
tonalites and diorites.
gabbros.
peridotites and other ultramafic rocks.
syenites, nepheline syenites and syenite pegmatites.
7. gneisses and schists.
8. metamorphosed limestones.
Heinrich, however has not made a distinction
between the igneous granites and the metamorphic-metasomatic granites. He has also not
studied the behaviour of biotites from charnockites.
Nockolds has studied the micas occurring in
the calc-alkali and the alkali igneous rocks. He
has arrived at the conclusion that the chemical
composition of biotites is a function of the
associated ferromagnesian minerals occurring in
that rock. He did not extend his theory to the
biotites occurring in the metamorphic-metasomatic rocks.
108
N. W.
Gokhale
Peikert has successfully demonstrated the
interdependence of the R.I. of a biotite and its
Fe:Mg ratio. By studying the areal distribution
of the R.I. values of biotites, he has come to
the conclusion that the granitic rocks containing them are of metasomatic origin.
Though in the studies referred to above,
biotite has been used to indicate the petrogenetic conditions of rock formation, the studies
have not been extended to distinguish between
identical crystalline rocks resulting from more
than one petrogenetic process. In view of this,
the author desirous of working further on biotites in respect of the chemical compositions of
biotites as an indicator of environment of the
formation of the rock in which they occur,
selected a number of chemical analyses of
biotites from the literature. They were processed
in order to plot them on different diagrams and
to establish whether there is any systematic
behaviour of these plots. The behaviour of
these plots and their usefulness in knowing the
environment of formation of the rocks in which
they occur, is discussed in this paper.
10 analyses from metamorphic-metasomatic
granites
(Peikert and Naik)
6 . . . do
acid charnockites of metamorphic origin
(Machigad)
16 . . . do
schists and gneisses
(Deer et al)
17 . . . do
igneous granites (Gokhale, Nockolds, Deer et
al)
The calculated values are tabulated in table 1.
Description of variation diagrams
Diagram
1
Methodology and materials used
The chemical analyses of biotites used for the
present study have been taken from Nockolds
(1947), Naik (1965), Peikert (1963), Deer et al
(1962), Machigad (1967), and Gokhale (1967).
These biotites are from calc-alkali granites,
alkali granites, potassic granites, acid charnockites and from metamorphic schists and gneisses.
The processed chemical data have been plotted
on three types of variation diagrams: one after
Heinrich, wherein Fe 2 O s + T i 0 2 : MgO : FeO
+ MnO values are taken into account; another
after Nockolds in which A1 2 0 3 , MgO and total
iron as FeO are selected as the critical constituents and the third one proposed by the present
author, wherein S i 0 2 , K 2 0 and N a 2 0 are taken
as the coordinates. In these calculations 49
biotite analyses are used, the details of which
are as under:
®
0
A
Biotites from igneous rocks: calc-alkali, alkali &
potassic granites.
Biotites from metamorphic-metasomatic rocks:
granites.
Biotites from metamorphic-metasomatic rocks:
schists & gneisses.
Biotites from metamorphic-metasomatic rocks:
acid charnokites.
Zone drawn as per Heinrich.
Separates biotites of magmatic environment from
those of metamorphic-metasomatic environment.
This has been constructed on the lines as suggested by Heinrich. He has demarcated the field
of plutonic biotites from that of the volcanic
Chemical composition of biotites as a guide
TABLE
Biotite
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
u
CA
0J
abe
T3
C
rt
m
JS
t/i
U
r^
Cu
O
cc
rt
3
0
<u
C
SC1
1—
S i»
o u
6
V c
o
•53 cs
b0
Cli
rC u
uO O
S
£ g
ci G
ti O
1
S
43.
44.
45.
46.
47.
48.
49.
56.17
49.9 5
57.28
63.3 0
41.03
51.49
41.86
60.3 0
65.5 2
56.7 9
81.71
tn
U
S
rt
VI
ÖO
25.
26.
27.
28.
29.
30.
31.
32.
33.
37.
38.
39.
40.
41.
42.
Fe a O,+TiO s
MgO
55.53
42.5 4
u
S
17.
18.
19.
20.
21.
22.
23.
24.
34.
35.
36.
NOCKOLDS VALUES
26.10
44.6 6
65.91
1.
2.
3.
(O
<U
TJ "u
" 2
^ v.
rt
V
1.
HEINRICH VALUES
FeO + MnO
from
40.03
21.27
11.61
12.71
13.04
16.29
6.3 5
15.01
18.41
18.84
13.45
11.67
10.97
21.33
lO.oo
10.3 9
MgO
33.87
34.0 6
22.4 8
31.12
37.01
26.42
30.3 5
43.9 6
30.10
39.3 0
26.2 5
22.8 0
32.23
23.14
47.4 6
7.90
34.03
20.5 7
15.94
25.6 9
5.45
11.13
9.3 6
13.32
8.62
0.61
14.9 8
23.8 4
17.36
22.12
29.40
34.7 4
35.44
34.5 0
30.9 9
34.0 3
20.8 5
24.57
22.3 9
22.10
20.2 7
22.7 6
23.99
31.19
24.17
28.8 3
23.5 9
33.17
36.3 3
22.63
16.95
25.31
27.00
7.00
9.63
25.0 0
47.00
33.3 6
33.42
29.8 6
31.52
42.6 6
21.00
17.42
7.50
18.96
35.8 2
33.40
46.7 5
47.9 0
46.8 9
46.70
55.56
58.5 0
62.6 8
42.51
50.4 3
14.5 5
43.04
41.53
40.67
29.7 7
25.46
26.67
31.09
18.62
17.80
14.42
19.80
11.13
2.47
51.41
54.47
58.8 4
41.70
42.6 4
0.71
O.oi
11.57
17.60
17.84
20.8 2
12.5 3
47.28
42.02
40.3 9
38.23
40.6 8
15.31
20.0 7
23.9 3
18.03
18.92
27.3 3
20.2 8
28.8 2
16.74
34.31
24.0 6
42.40
63.7 8
64.3 6
64.7 6
67.67
70.7 8
67.13
81.87
68.06
48.41
52.42
65.05
48.00
46.0 0
41.21
37.00
40.6 4
21.81
24.4 5
12.46
9.02
10.35
18.36
12.14
10.39
10.78
17.31
12.25
14.74
23.8 3
15.60
13.20
13.76
71.58
79.6 8
72.63
42.10
36.0 7
37.3 6
40.18
34.7 6
37.9 7
37.5 6
30.9 4
29.7 8
31.03
17.69
12.90
25.5 3
26.3 7
14.90
14.40
14.67
19.32
27.2 8
65.00
74.8 5
59.7 3
49.80
69.5 0
72.4 0
Al.O,
18.11
30.4 0
17.52
16.96
1.01
0.12
8.12
7.30
8.0 3
8.8 5
4.72
44.6 0
41.29
37.5 0
7.02
30.3 0
32.31
36.7 8
36.92
37.88
34.97
37.3 6
36.8 6
ones by drawing lines parallel to the base MgO—
F e 2 0 3 + T i 0 2 which does not help to distinguish between the biotites of igneous and metamorphic-metasomatic rocks. However line
drawn parallel to the base FeO + MnO—
109
GOKHALE V A L U E S
Total iron
36.08
39.4 8
47.3 3
39.7 5
34.8 5
44.9 7
40.4 4
30.64
39.3 3
32.9 4
42.5 8
43.7 5
36.7 9
41.55
23.8 8
60.51
SiOj
K,O
79.61
79.49
79.09
80.4 4
14.86
19.70
19.99
79.9 6
77.56
81.04
80.4 8
78.4 9
80.6 2
78.18
78.54
79.08
79.77
81.92
78.30
50.61
56.20
51.68
44.09
57.82
59.8 4
78.54
81.10
58.19
64.9 8
75.84
78.77
80.6 6
46.03
45.83
81.87
80.2 3
79.3 6
79.47
76.17
78.21
79.6 6
19.16
19.69
21.40
18.01
16.83
20.6 2
19.23
21.35
19.32
0.3 5
1.04
0.97
2.69
0.89
0.15
0.47
2.14
0.84
1.53
1.42
0.31
20.14
17.75
16.76
18.33
19.47
19.59
20.6 5
15.75
1.32
1.15
1.37
1.44
20.17
20.8 5
3.67
0.92
1.22
19.12
18.95
21.38
18.10
1.17
0.94
0.57
3.58
19.63
18.86
10.28
0.99
0.82
0.33
3.35
0.84
0.75
16.73
18.14
17.14
0.74
0.47
0.44
15.93
15.64
19.01
18.62
16.80
15.56
0.68
72.8 6
57.3 8
43.71
40.9 3
82.53
81.39
43.5 8
44.20
82.42
83.39
46.71
45.64
28.37
49.16
62.6 3
49.5 2
28.97
83.41
80.5 5
80.8 6
82.8 6
79.95
29.7 5
30.95
29.5 5
25.20
22.45
23.7 9
33.23
37.0 4
36.92
42.5 7
38.85
77.9 3
75.44
80.15
80.13
77.93
18.57
20.5 8
17.45
17.44
20.5 2
20.15
42.9 9
78.06
18.89
2.05
0.49
11.53
5.5 3
0.81
0.92
0. 40
20.0 8
18.70
16.66
21.39
80.0 5
77.80
81.56
77.02
80.28
88.9 4
49.11
46.03
61.62
72.49
Na a O
16.76
0.94
0.44
0.52
0.34
4. 52
3.4 9
3.40
3.9 8
2.40
2.43
1.55
3.05
F e 2 0 3 + T i 0 2 clearly separates the biotites of
metamorphic-metasomatic rocks from those of
igneous rocks. It is interesting to note that the
biotites from charnockites fall into the field of
metamorphic biotites, a point not tested by
110
N. W.
Gokhale
Heinrich, which is apparent from the groups of
rocks considered by him. Further he has treated
only the biotites of igneous granites. The present
study reveals that the biotites from the metamorphic-metasomatic granites have clearly separated from those of igneous granites (diagram 1).
A plot or two not occupying the field concerned,
constitute exceptions.
Diagram
2
This has been constructed on the lines suggested by Nockolds. This diagram was primarily
designed by him to bring out the interdependence of the chemical composition of biotite and
the nature of the associated ferromagnesian minerals occurring along with it in that rock.
Nockolds restricted his observations to the biotites of igneous rocks. Plotting the chemical
data of the biotites of metamorphic-metasomatic
rocks too, it is seen (diagram 2) that such
#
0
a
«
Biotites from igneous rocks: calc-alkali, alkali &
potassic granites.
Biotites from metamorphic-metasomatic rocks:
granites.
Biotites from metamorphic-metasomatic rocks:
schists & gneisses.
Biotites from metamorphic-metasomatic rocks:
acid charnokites.
Zone demarcated by Nockolds for igneous rocks.
Separates biotites of magnatic environment from
those of metamorphic-metasomatic environment.
biotites occupy the same field as the one occupied
by biotites of igneous rocks, eventhough the
associated ferromagnesian minerals are entirely
different from those found in metamorphic
-metasomatic rocks. This diagram however, can
be employed to distinguish between the biotites
of igneous and metamorphic-metasomatic origin. This can be achieved by drawing a line
parallel to the base total iron as FeO—A1 2 0 3 .
Such a line drawn on diagram 2 is seen clearly
demarcating the plots of biotites formed under
metamorphic-metasomatic environment from
those formed under magmatic environment.
Diagram
3
This diagram was proposed by the present
author (1967) in an earlier article. The three
oxides — S i 0 2 , K 2 0 and N a 2 0 —were selected
as the critical constituents, since it is largely
believed that these are the major constituents
operative in the process of granitisation. As a
consequence, it is reasonable to presume that
these three oxides might be at least influencing
the chemical composition of biotites formed
by metamorphic-metasomatic processes. Surprisingly however, it is immediately noticed
from diagram 3, that the biotites formed under
different environments are practically identical
in respect of the contents of SiO a , K 2 0 and
Na a O.
*
^
Biotites from igneous rocks: calc-alkali, alkali &
potassic granites.
Biotites from metamorphic-metasomatic rocks:
granites.
Biotites from metamorphic-metasomatic rocks:
schists & gneisses.
Biotites from metamorphic-metasomatic rocks:
acid charnokites.
Chemical composition of biotites as a guide
Conclusions
From the foregoing account it is evident that
the mineral biotite, plays an important role in
distinguishing magmatic and metamorphic-metasomatic rocks and especially the granites. The
present study has conclusively proved that the
chemical composition of biotites formed under
different environments is different. Heinrich
eventhough regarded the chemical composition
of biotites to indicate the geological occurrence,
he however did not distinguish further between
the fields of igneous and metamorphic-metasomatic granites. In the present study it is
shown, how the processing of chemical data on
the lines of Heinrich and on those of Nockolds,
can be extended to metamorphic-metasomatic
granites and also to charnockites.
Peikert's conclusion that the granites are of
metasomatic origin is also arrived at by the
present author, by merely processing the chemical composition of biotites occurring in those
rocks, on the lines set out in this paper.
Based on several evidences, Naik (1965) advocates a metamorphic-metasomatic origin to the
Molakalmuru granites. The biotites occurring in
these rocks when treated on the lines given in
this paper, it is seen that the biotites and the
111
rocks containing them are formed under metamorphic-metasomatic environment. As already
noted, diagram 3 does not show any distinction
between the biotites formed under different
environments. This phenomenon is especially
strange while dealing with the granitised granites,
since in this process the major event is one of
introduction of potassium into the preexisting
rocks. It appears that apparently this action has
not affected the K 2 0 content of the biotites of
such rocks.
Thus it is concluded that the chemical composition of biotites, fluctuates according to the
environment of its formation. Chemical evidence
by way of K-metasomatism fails to be an index
to the genesis of the granites, if the K 2 0 content together with those of S i 0 2 and N a 2 0 of
the biotites occurring in such rocks are utilized.
Peikert has already utilized biotite as a guide to
ascertain the origin of granites. The present
studies have also indicated that biotite certainly
could be looked at as an index mineral in pedogenesis, more so with the granites, since it is the
only ferromagnesian mineral occurring in them.
Further studies however, are necessary, since
the present author has based his observations on
only 49 chemical analyses of biotites.
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Manuscript received, February 22, 1968.